That's funny....I"m trying the same experiment on my wife
I think the distro you choose depends on your goals. If you just want your wife to use linux, but not really do any of the admin work, I'd stick with Slackware. That 's what I'm doing. I've got a laptop set up to boot to kdm and I set her up with KDE as the X environment. It's close enough to windows that she can operate pretty successfully.
However if you want her to do some of the admin work, I'd still stick with Slackware. Odds are she is going to be dealing with compiled packages for software installation and how hard is it to use pkgtool?